Typically, overlapping steel sheets are joined by resistance spot welding which is one type of lap resistance welding.
Resistance spot welding is a method of squeezing two or more overlapping steel sheets by a pair of electrodes from above and below and, while applying an electrode force, passing a high welding current between the upper and lower electrodes for a short time to join the steel sheets. Heat generated from the resistance to the flow of the high welding current is utilized to obtain a spot weld. The spot weld is called a nugget, and results from the overlapping steel sheets melting and solidifying at their contact portion when the current flows through the steel sheets. The steel sheets are thus spot-joined.
For good weld quality, it is important to form the nugget with a diameter in an appropriate range. The nugget diameter depends on the welding condition such as welding current, welding time, electrode shape, and electrode force. To achieve an appropriate nugget diameter, the welding condition needs to be set appropriately according to the parts-to-be-welded condition such as the material, sheet thickness, and number of overlapping sheets of the parts to be welded.
In vehicle manufacturing, for example, spot welding is performed at several thousand points per vehicle, and parts to be welded (workpieces) conveyed one after another need to be welded. Here, if the parts-to-be-welded condition such as the material, sheet thickness, and number of overlapping sheets of the parts to be welded is the same at each welding location, the same welding condition such as welding current, welding time, and electrode force can be used to obtain the same nugget diameter. In continuous welding, however, as welding is performed multiple times, the contact surfaces of the electrodes with the parts to be welded wear gradually and the contact areas between the electrodes and the parts to be welded widen gradually. When the same welding current as in the initial welding is passed in such a state where the contact areas have widened, the current density in the parts to be welded decreases and the temperature rise of the weld drops, which leads to a smaller nugget diameter. Accordingly, the electrodes are dressed or replaced every several hundred to several thousand welding points, to prevent the electrode tip diameter from increasing excessively.
A resistance welding device having a function (stepper function) of increasing the welding current after a predetermined number of welding operations to compensate for a decrease in current density associated with electrode wear has been conventionally used, too. To use the stepper function, the aforementioned welding current change pattern needs to be set appropriately beforehand. This, however, requires considerable time and cost as the welding current change patterns corresponding to numerous parts-to-be-welded conditions and welding conditions need to be derived through tests and the like. Besides, since the state of progress of electrode wear varies in actual work, the welding current change pattern set beforehand may not always be appropriate.
In the case where disturbances are present during welding such as when a point that has already been welded (existing weld) is present near the current welding point or when the parts to be welded have significant surface roughness and a contact point of the parts to be welded is present near the welding point, part of the current diverts into such an existing weld or contact point during welding. Even when welding is performed under a predetermined condition in this state, the current density at the position to be welded which is directly above and below the electrodes decreases, and so a nugget of a required diameter cannot be obtained. To compensate for such an insufficient amount of heat generated and obtain a nugget of a required diameter, a high welding current needs to be set beforehand.
Moreover, in the case where the surroundings of the welding point are firmly restrained due to surface roughness, member shape, etc., a larger sheet gap between the steel sheets causes a smaller contact area of the steel sheets, which may hinder the obtainment of a nugget of a required diameter or facilitate splashing.
The following techniques have been proposed to solve the problems stated above.
For example, JP H9-216071 A (PTL 1) describes a control unit of a resistance welder that compares an estimated temperature distribution of a weld and a target nugget and controls the output of the welder to obtain the set nugget.
JP H10-94883 A (PTL 2) describes a welding condition control method for a resistance welder of detecting the welding current and the voltage between tips, simulating a weld through heat transfer calculation, and estimating the nugget formation state to achieve good welding.
JP H11-33743 A (PTL 3) describes a welding system that calculates, from the sheet thickness of parts to be welded and the welding time, the cumulative amount of heat generated per unit volume with which good welding of the parts to be welded is possible, and adjusts the welding current or voltage to generate the calculated amount of heat per unit volume and per unit time. According to PTL 3, the use of the welding system enables good welding regardless of the type of the parts to be welded or the wear state of the electrodes.
JP 2004-58153 A (PTL 4) describes a welding method of calculating a curve that takes the value of the power and the current or the square of the current per half cycle during current passage, determining the nugget formation state based on the change of the curve, and either adjusting the current or the electrode force in the subsequent cycles or terminating the current.